1. Field of the Invention
The present invention relates to a air-fuel ratio control device of an internal combustion engine.
2. Description of the Related Art
An internal combustion engine is known, which comprises an electric purge control valve for controlling the supply of purge gas fed into the intake passage of an engine from a charcoal canister, and an electric air bleed control valve for controlling the amount of air fed into the fuel passage of a carburetor. An electric current fed into the air bleed control valve is controlled on the basis of the output signal of an oxygen concentration detecting sensor (hereinafter referred to as an O.sub.2 sensor) arranged in the exhaust passage of the engine so that the amount of air fed into the fuel passage of the carburetor is increased as the amount of electric current fed into the air bleed control valve is increased (Japanese Unexamined Patent Publication No. 61-1857). In this engine, when the purge control valve is opened, and thus the supply of the purge gas is started, if the purge gas contains a large fuel component, an air-fuel mixture fed into the engine cylinders becomes extremely rich. As a result, the amount of electric current fed into the air bleed control valve is gradually increased so that an air-fuel ratio approaches the stoichiometric air-fuel ratio, and accordingly, the amount of air fed into the fuel passage of the carburetor is gradually increased. Subsequently, when the electric current fed into the air bleed control valve is increased to the maximum level of the controllable range, an air-fuel ratio control is changed from the air-fuel ratio control based on the air bleed control to the air-fuel ratio control based on the purge control, and thus the amount of purge gas is controlled so that the air-fuel ratio approaches the stoichiometric air-fuel ratio.
However, actually, when the supply of purge gas is started, the electric current fed into the air bleed control valve normally does not reach the maximum level of the controllable range, and thus, at this time, the amount of air fed into the fuel passage of the carburetor from the air bleed passage is gradually increased until the air-fuel ratio of air-fuel mixture fed into the engine cylinders becomes equal to the stoichiometric air-fuel ratio. However, if the amount of air fed from the air bleed passage is gradually increased as mentioned above, it takes a long time to equalize the air-fuel ratio with the stoichiometric air-fuel ratio. Consequently, since an extremely rich air-fuel mixture is still fed into the engine cylinders for a long time, a problem occurs in that a large amount of unburned HC and CO is discharged from the engine cylinders during that time.
A fuel injection type engine having a charcoal canister is also known. The charcoal canister comprises a fuel vapor outlet connected to the intake passage in the vicinity of the throttle valve, and an air inlet connected to the intake passage upstream of the throttle valve and downstream of the air flow meter (Japanese unexamined Utility Model publication No. 61-13735). In this engine, when the throttle valve is open, the fuel vapor outlet of the charcoal canister is connected to the intake passage downstream of the throttle valve. Consequently, at this time, a part of air metered by the air flow meter is fed into the charcoal canister, and thus the fuel component adsorbed in the activated carbons is desorbed by this air. The fuel component thus desorbed is then fed into the intake passage.
In addition, another fuel injection type engine having a charcoal canister is known, wherein the charcoal canister comprises a fuel vapor outlet connected to the intake passage in the vicinity of the throttle valve, and an air inlet selectively connected to the outside air or the intake passage upstream of the throttle valve and downstream of the air flow meter via a control valve (Japanese unexamined Utility Model publication No. 58-64854). In this engine, when the engine is stopped, the air inlet of the charcoal canister is connected to the outside air so that an excess fuel component which can not be adsorbed by the activated carbons can be discharged to the outside air but not to the intake passage, when a large amount of fuel vapor is generated in the fuel tank. Conversely, when the engine is operating, the air inlet of the charcoal canister is connected to the intake passage between the throttle valve and the air flow meter. Consequently, when the throttle valve is open, a part of air metered by the air flow meter is fed into the charcoal canister, and the fuel component desorbed from the activated carbons is fed into the intake passage.
Note, in the above-mentioned fuel injection type engines, the injection time TAU of the fuel injector is determined basically on the following equation. EQU TAU=TP.multidot.FAF
In this equation, TP indicates a basic injection time determined by both the engine speed and the amount of air fed into the engine cylinders, and FAF indicates a feedback correction coefficient changed on the basis of the output signal of the O.sub.2 sensor so that an air-fuel ratio becomes equal to the stoichiometric air-fuel ratio. This FAF normally varies around 1.0 and, to prevent the FAF from becoming excessively large or excessively small, an upper guard and a lower guard are provided for the FAF. The upper guard is, for example, 1.2, and the lower guard is, for example, 0.8. When the FAF is increased and reaches the upper guard, that is, 1.2, the FAF is maintained at 1.2, and when the FAF is reduced and reaches the lower guard, that is, 0.8, the FAF is maintained at 0.8. Therefore, the FAF is able to vary between 0.8 and 1.2.
In the above-mentioned fuel injection type engines, when the supply of purge gas from the charcoal canister into the intake is started, if the purge gas contains a large fuel component, an air-fuel mixture fed into the engine cylinders becomes extremely rich. At this time, the FAF is reduced to the amount of fuel injected from the fuel injection. However, at this time, the FAF sometimes reaches the lower guard and is maintained at 0.8. In this case, the feedback operation of the air-fuel ratio is no longer carried out, and the air-fuel mixture remains rich, and as a result, a problem occurs in that a large amount of unburned HC and CO is discharged from the engine cylinders.